1. Field of the Invention
The present invention relates to a flexible joint and a long nut for flexible joint, and more particularly, to a telescopic flexible tube joint which is used for connecting a water pipe and is capable of absorbing external force even if excessive external force is applied and thus, is capable of preventing the tube from being destroyed, and is easily handled, and to a long nut used for such a telescopic flexible tube joint.
2. Description of the Related Art
Tubes such as long water pipes laid on or under the ground are subjected, in some cases, to various external force such as compression force and tensile force in a direction of the core of the tube shaft due to ground subsidence or earthquake, and shearing force and bending moment. If such external force is too great, stress is concentrated on a tube joint which connects the tubes, and the tube joint is destroyed. Thereupon, it is necessary to absorb the external force applied to the water pipe portion to prevent the tube joint from being damaged.
Conventionally, in order to absorb the external force to prevent the destruction, there has been developed a tube joint which is telescopic by its own. However, since this tube only telescopically moves, the movable region is two dimensional, and this tube can not sufficiently cope with the external force due to ground subsidence or earthquake.
To solve such a problem there has been developed a tube joint having three dimensional flexibility by employing the structure for bringing spherical surfaces into contact with each other for sliding movement. However, since the tube joint itself has the telescopic flexibility, when the water pressure test is carried out prior to use or when the tube joint is being transported and moved, various members constituting the joint are moved by vibration. For this reason, it is troublesome to handle the tube joint before the tube joint is connected to the tube, and it is not convenient for actual use.
Thereupon, there has been developed a joint provided with preventing means for preventing the relative movement of cylinders relatively movably connected to each other (e.g., Japanese Patent Application Laid-open No. H8-121665). This joint has structure that a pair of casing tubes constituting the joint are provided at their outer peripheral surfaces with bosses, a rod is provided between the pair of casing tubes, and a long nut having a through hole in its center is fitted into a substantially center portion of the rod in its longitudinal direction. This long nut is formed with a notched portion, and when great external force applied to the joint, this notched portion is broken so that the flexible telescopic characteristics can be exhibited.
However, in this structure, since the long nut used here is provided with the through hole and the rod is threaded into the through hole, there is a possibility that the long nut is not necessarily be constantly located over the longitudinal direction of the joint. That is, two to four rods are disposed around the tube joint and position of each rod is not necessarily constant, and even if the rods are initially located at constant positions, there is a probability that the long nut may move during transportation. Further, locating the rod at constant position itself is troublesome. As a result, when great external force is applied to a tube joint which is buried in the ground, there is a possibility that the long nut is not reliably broken from its notched portion, the timing for exhibit the inherent telescopic flexibility is delayed, and the tube joint itself or the tube is damaged.
Thereupon, it is an object of the present invention to provide a joint which has a great telescopic flexibility so that the drawbacks of the conventional joints can be overcome and the joint can sufficiently cope with the excessive external force applied due to the ground subsidence or earthquake, and which has substantially constant breaking position of the rod so as to reliably maintain the inherent telescopic flexibility, and position of a long nut is not deviated easily during transportation, and which is convenient for handling.